Ion and electron heating at collisionless shocks near the critical Mach number

Abstract

The evolution of the ion and electron distribution functions across a set of 10 low‐Mach number, nominally subcritical, quasi‐perpendicular shocks is examined in high time resolution (full distribution every 3 s) using data from the ISEE 1 and 2 spacecraft. Both ions and electrons sometimes show slight preheating upstream of the shock, but otherwise the ion and electron temperatures rise together in the magnetic ramp and show no further increase downstream. Contrary to the usual assumption based on early laboratory and theoretical work that at subcritical shocks the bulk of the energy dissipation occurs as resistive heating of the electrons, it is found that the ion temperature increase exceeds that of the electrons. This difference is attributed to the distinction between dispersive shocks, such as those studied here, and resistive shocks, such as those observed in most laboratory studies. The increase in ion temperature is predominantly in the perpendicular direction and is due to heating of the entire distribution rather than to the formation of a high‐energy tail. The perpendicular temperature increase is typically a factor of 10–20, much greater than the usual assumption of adiabatic heating. The downstream to upstream ratio of perpendicular electron temperature is equal to the magnetic field ratio (∼2–2.5). The electrons also show significant heating in the parallel direction, with the downstream T∥/T⊥ ∼1–1.2. The downstream electron distribution exhibits the characteristic flattop seen downstream of supercritical shocks, and there is evidence for the field‐aligned electron beam identified previously within those shocks. As previously reported, the downstream ion and electron total temperatures are nearly equal. These observations are interpreted as evidence for the simultaneous operation of several plasma instabilities, including the modified two‐stream instability, driven by the cross‐field current within the shock, and the ion acoustic instability, driven by the field‐aligned electron beam.

Magnetic field fluctuations across the Earth’s bow shock

“…An important role in the dissipation process is played by ions reflected at the bow shock (Sckopke et al, 1983;Thomsen et al, 1985). At quasi-parallel shocks, they can escape from the shock into the foreshock region and drive ion beam instabilities.…”

Electromagnetic proton/proton instabilities in the solar wind: Simulations

“…The standard view of wave-particle interactions is that enhanced fluctuations from an instability scatter particles to reduce the anisotropies which lead to wave growth. Thomsen et al [1985]), heating by instabilities driven by interplanetary pickup ions , and heating by proton/proton instabilities [Schwartz et al, 1981]. Gary et al [1986] showed that the magnetosonic instability at n• • nc does not induce a significant core anisotropy, but until now there have been no simulation studies to determine whether the Alfv•n instability is a plausible source of this anisotropy.…”

Ion and electron heating at the Martian bow shock. Common for bow shocks or not?

“…Earlier studies on the ion dynamics at the Earth's bow shock point out that the following features are commonly observed for both critical and supercritical shocks (Thomsen et al, 1985;Sckopke et al, 1990 and references in these papers): (1) ions reflected from the shock rump and accelerated by the solar wind electric field in the foot region; (2) sharp broadening of ion distribution (heating) at the shock rump; (3) gyrating ions in the downstream region; (4) their relaxation leading to the high energy non-Maxwellian tail in the depth of downstream region; (5) the observed ratio of proton heating exceeds the adiabatic level. These phenomena typically observed in the near-Earth space resemble those presented in this paper.…”